The present invention relates to a constant speed control apparatus for a motor vehicle, which can automatically control a speed without requiring operation of an accelerator pedal and, more particularly, to a constant speed control apparatus for a motor vehicle, which causes an automatic transmission to shift down when a detected vehicle speed is decreased by a predetermined value or more from a target vehicle speed during vehicle travel.
As a conventional constant speed control apparatus for a motor vehicle which causes an automatic transmission to shift down when a detected vehicle speed is decreased by a predetermined value or more from a target vehicle speed during travel, a technique described in, e.g., Japanese Patent Laid-Open (Kokai) No. 57-19631 is known. In this conventional technique, a shift-up timing from the shift-down state is set at a time after the lapse of a predetermined time interval from when the detected vehicle speed is recovered to a target vehicle speed.
The conventional technique assures comfortable constant speed travel even on an uphill. However, if the timer sets a short predetermined time interval, shift-up and shift-down operations are repeated several times on a long uphill, thus impairing comfort. In the conventional technique, if the timer sets a long predetermined time interval, when a shift-down operation is performed on a short uphill, the shift-down state is maintained for a long period of time even if the road changes to a downhill, thus posing a problem in terms of an improvement of fuel consumption.